People with the most Facebook "friends" are more likely to feel stressed out by the site, according to researchers.

Edinburgh Napier University found a significant minority of users suffered "considerable Facebook-related anxiety".

However, they only received very modest rewards.

More than one in 10 said Facebook made them feel anxious and more than three in 10 said they felt guilty about rejecting friend requests.

The study found that 12% of the students questioned said they disliked receiving friend requests, while almost two thirds (63%) said they delayed replying to friend requests.

The university's Dr Kathy Charles, who led the study, said: "The results threw up a number of paradoxes.

"For instance, although there is great pressure to be on Facebook there is also considerable ambivalence amongst users about its benefits.

"And we found it was actually those with the most contacts, those who had invested the most time in the site, who were the ones most likely to be stressed."

Like gambling, Facebook keeps users in a neurotic limbo, not knowing whether they should hang on in there just in case they miss out on something goodDr Kathy Charles, Edinburgh Napier University

Dr Charles said this could be because of the pressure users feel to come up with updates about their life for a large number of people.

She said: "It's like being a mini news channel about yourself. The more people you have the more you feel there is an audience there.

"You are almost a mini celebrity and the bigger the audience the more pressure you feel to produce something about yourself."

Dr Charles added that an "overwhelming majority" of respondents reported that the best thing about Facebook was "keeping in touch" but many people said they were anxious about withdrawing from the site for fear of missing important social information or offending contacts.

"Like gambling, Facebook keeps users in a neurotic limbo, not knowing whether they should hang on in there just in case they miss out on something good," she said.

The survey also found that other causes of tension included purging unwanted contacts, the pressure to be inventive and entertaining, and having to use appropriate etiquette for different types of "friends".

The researchers quizzed about 200 students on their use of the site, which now has more than 500 million users worldwide.

They used focus groups, an online survey of 175 people and one-to-one interviews to collect the data.